Abstract: Thus paper reports the study on the average decreasing amplitude of leprosy prevalence of Yangzhou city in comparison yvith parasnetera a, b of exponential function models. The results indicate that the parametEr b reflects directly the decreasing rates of leprosy preva. lence. This indicator is even more accurate, sensitive and can express the real reduction of the prevalcnce than the average reductive rate. Through analysing the relationship between the decreasiing amplitude and leprosy prevalence rate we find that the decreasing amplitude of leprosy is positively correlated to the prcvalcnct at the early period, but not to the prevalence at the later period.